Elevated pressure cryogenic air separation plants are widely known in industry, for example, in Gasification of Carbonaceous Compounds applications. In an elevated pressure plant, the lowest pressure distillation column still operates at a pressure higher than the ambient pressure. As a consequence, all the gaseous products leave the distillation system at elevated pressure. One of the common products of an air separation plant, in addition to nitrogen, oxygen and argon, is a waste stream used to regenerate the front end adsorption bed. The waste stream is produced from the column operating at the lowest (still elevated) pressure. Since the waste stream is eventually vented to atmosphere, its pressure does not need to be much higher than the atmospheric pressure. The objective of the present invention is to provide an efficient solution for utilization of the excess pressure energy contained in the waste stream.
Attempts to recover excess pressure energy from waste have been made for single column plants or single product plants. Single column air separation plants have low recovery and, therefore, they generate a relatively high amount of waste. Also, a relatively high amount of waste gas is produced in single or multiple column plants design to recover only one air component, for example, in nitrogen generators. The obvious form of utilizing the excess pressure energy contained in waste is turbo-expanding the waste stream with the creation of external work. This also provides refrigeration necessary for the operation of a cryogenic plant. Examples of single- and multiple-column nitrogen generators with waste expanders are described in several patents and other publications, for example, U.S. Pat. Nos. 4,439,220, 4,448,595, 4,453,957, 4,927,441 and 5,098,457. It is not indicated in these patents whether the external work generated by the waste expander is in any way recovered or not.
The external work created in the waste expander can be dissipated to the environment in the form of heat. This is fairly inefficient, but it is used when expander power is small and the profit from power recovery would not justify the capital cost of the recovery system, for example, electric power generator. In this case, the only objective of the expander is to supply the necessary refrigeration for the plant.
The use of an electric power generator, although more efficient than dissipating the power, does not provide the highest possible efficiency either because of thermodynamic losses in the generator. Furthermore, potential synergistic thermodynamic benefits of loading the power on a process compressor (compander) are not realized.
The power generated by the waste expander can be at least in part utilized to compress another process stream in a compander. This solution was applied to single column nitrogen generators described in U.S. Pat. Nos. 4,966,002 and 5,385,024. In these cycles, a portion of an oxygen enriched waste stream was expanded to compress another portion of the same waste stream that is recycled back to the distillation system. Since the compression occurs at a cryogenic temperature, the absolute compression power is not high, but the need for additional refrigeration increases significantly. Therefore, the external power generated by the waste expander is only partially recovered in an associated compander and a significant part of this power is dissipated.
Cycles composed of two or more distillation columns, where nitrogen and oxygen are produced, usually have high recovery and the waste stream is relatively small. If it is expanded at a cryogenic temperature, the power extracted from the cold side of the system can merely provide required refrigeration for a plant (the detailed result depends also on the pressure ratio). Because of the low expander temperature and small waste flow, this power alone is often not sufficient in practice to compress any other process streams. If the process stream is compressed at a cryogenic temperature, cold compression, the increased refrigeration need of the cycle cannot be met by this waste expander alone. In the case of warm compression, the refrigeration demand does not increase, but the absolute value of power of the cold expansion is usually not very significant when transferred to the warm end of the plant. This is probably the reason that the concept of recovering the waste expander power to compress a process stream has not yet been widely used in a multiple column system. Two exceptions have been described in U.S. Pat. No. 4,072,023 and EP-0,384,483.
In EP-0,384,483 a portion of a waste stream is expanded to create the desired refrigeration. At the same time, the work of expansion is utilized in a compander compressor to warm compress another portion of the same waste stream. Since, as discussed, the expansion power was not significant, an external compressor had to be used in addition to the compander.
U.S. Pat. No. 4,072,023, by Springmann, describes an air separation process consisting of a reversing heat exchanger and a double column system. A reversing heat exchanger has been used in air separation to purify the feed air stream from high boiling components (such as water vapor and carbon dioxide), by freezing these components inside the heat exchanger passages. Springmann expressly teaches to use excess refrigeration to cold compress a suitable process stream having a temperature no warmer than the temperature of the cold side of the main heat exchanger.
It is widely known that the waste stream in elevated pressure cycles can be heated to a high temperature and expanded to generate electrical power. A disadvantage of hot gas expansion is the very high capital cost of the entire hot expansion and power generation system.
The scale of air separation plants is growing continuously and also the absolute value of the waste flow rate (as well as all the other flow rates) is high in large plants. Therefore, the pressure energy of the waste stream and other process streams is fully capable to provide more than necessary refrigeration for the plant. This excess refrigeration can be used to liquefy the products of air separation (U.S. Pat. No. 5,165,245) or to decrease the size of the main heat exchanger (U.S. Pat. No. 5,146,756).